docs: def _handle_paired_with(self, paired_with):
          '''Handle input option paired_with'''
          if paired_with is None or not paired_with:
              var_name = []
              var_value = []
          elif isinstance(paired_with, str):
              var_name = ['_' + paired_with]
              if paired_with not in env.sos_dict:
                  raise ValueError(f'Variable {paired_with} does not exist.')
              var_value = [env.sos_dict[paired_with]]
          elif isinstance(paired_with, dict):
              var_name = []
              var_value = []
              for k, v in paired_with.items():
                  var_name.append(k)
                  var_value.append(v)
          elif isinstance(paired_with, Iterable):
              try:
                  var_name = ['_' + x for x in paired_with]
              except Exception:
                  raise ValueError(
                      f'Invalud value for option paired_with {paired_with}')
              var_value = []
              for vn in var_name:
                  if vn[1:] not in env.sos_dict:
                      raise ValueError(f'Variable {vn[1:]} does not exist.')
                  var_value.append(env.sos_dict[vn[1:]])
          else:
              raise ValueError(
                  f'Unacceptable value for parameter paired_with: {paired_with}')
          #
          for vn, vv in zip(var_name, var_value):
              # set paired with values to step_input
              self.paired_with(vn, vv)